The present invention relates to the detection of the presence of a device on a high speed serial data bus and in particular the detection of a xe2x80x98physical layer devicexe2x80x99 on a serial management interface bus driven by a station management entity in a manner conforming to the operational standard, IEEE 802.3 relating to ethernet networks. One object of the invention is to provide for improved detection of a physical layer device while conforming to that standard and particularly clause 22 thereof but the invention is not necessarily limited to that context. In particular it may be suitable for operating a similar interface at higher rates.
As will be explained in more detail with specific reference to the aforementioned standard, a preferred and exemplary form of the invention provides for the detection of the presence of a physical layer device which can accept serial data from and provide serial data to a station management entity with which it constitutes a media independent interface within the general xe2x80x98physicalxe2x80x99 layer (according to the OSI reference model) between a transmission medium for the conveyance of signals defining frames or packets and the data link or media access control layer of a network device such as, for example, a switch or hub.
A physical layer device of this kind (usually abbreviated to xe2x80x98PHYxe2x80x99) requires management so that a control and monitoring device, otherwise known as the station management entity (xe2x80x98STAxe2x80x99) can configure the physical layer device, so far as may be necessary or possible, to determine the operational state or mode for the physical layer device. This is achieved according to the aforementioned standard by, as will be more fully explained below, the use of registers within a state machine of the physical layer device, the registers including at least one control register into which the station management entity may write and at least one status register from which data may be read back to the station management entity. As their names imply, the control register is employed for setting or altering the operational state or mode of the physical layer device whereas the status register indicates the actual operational state or mode. The control register determines, for example, whether the physical layer device is in normal operation or should be reset, whether or not it is in loop-back mode, what its operational speed may be, whether auto-negotiation is enabled, whether it is to operate in full duplex or half duplex mode so on. The status register generally indicates whether the physical layer device is able to perform in specified transmission modes (such as half duplex or full duplex), and at which speeds and indicates other modal characteristics not primarily relevant to the present invention.
In the case of a multi-port network device it is customary to provide for management of a multiplicity of such physical layer devices by a common station management entity and, will be indicated, the management control by that entity is exercised by the transmission of defined serial data words which include data fields for addressing the individual physical layer devices and specific registers within them as well as determining whether a write operation or a read operation is to be performed. The Media Independent Interface (MII) Management Interface conforms to various requirements set out in the mu aforementioned standard and, at the present time, is intended to be capable of accommodating up to thirty-two physical layer devices with a single station management entity, subject to certain restrictions not relevant to the present invention. Physically the management is exercised by means of data conveyed over a serial bus through the MII Management Interface to the physical layer devices, the bus normally being termed the management data input/output (MDIO) bus. Another line conveys a clock signal, the management data clock (MDC), since it is necessary that the management data be properly synchronized to that clock. A station management entity and a physical layer device each have state machines which provide information to a respective line driver coupled to the MDIO bus and receive data from the bus by way of a buffer amplifier. How the signals are generated and the particular internal workings of the state machines are not fundamental to the present invention, the aforementioned standard specifies in some detail the physical constraints on the signals to be employed.
One function which is desirable to perform is the detection of whether a particular physical layer device is actually coupled to the bus. It is desirable to allow products having for example different numbers of ports (and therefore different numbers of PHYs) to have the same design of STA and the same software. It is accordingly desirable to be able to determine whether particular PHYs are fitted or not. It is also desirable to be able to detect the xe2x80x98hotxe2x80x99 removal of a device from the system and to detect modules that may or may not be present.
The signal sent to a physical layer device by the management entity, usually termed xe2x80x98framexe2x80x99 has a defined format and is required to be placed on and read from the data bus by means of tri-state devices. More particularly, the frame format includes after the data field defining the address data, a xe2x80x98turnaroundxe2x80x99 portion, constituted by two bit times in the standard. During a read transaction the first bit time is provided to ensure that there is no contention with other signals during the change over from the STA""s driving of the line to the PHY""s driving of the line. The second bit time is provided to indicate a response by an addressed device. The defined states of the station management entity and the physical layer device during the turnaround time are such as to enable the station management entity to determine whether the addressed physical layer device is present by monitoring the potential of the data bus during that time. The use of the aforementioned standard for xe2x80x98multi-dropxe2x80x99 busses requires a resistive pull-up for the data bus, normally implemented by a 1.5 kilohm resistor between the bus and the positive rail (at 5 volts). This necessary feature will provide that if during a xe2x80x98readxe2x80x99 access an addressed PHY is present, and responds, the line will be xe2x80x98lowxe2x80x99 during the second turn-around bit, whereas if there is no device at the PHY address which is accessed, the line will float xe2x80x98highxe2x80x99. The voltage level of the bus can be monitored by way of the input buffer of the station management entity, so as to perform a comparison of the voltage level of the bus with a reference and to provide an indication whether the physical layer device is present or not.
Since however the pull-up resistor is associated with stray capacitance, there is in a variety of circumstances a delay, determined by the time constant of the circuit branch including the resistor, before the voltage level reaches a substantially steady state which gives a reliable indication whether the addressed physical layer device is present or not. Accordingly, as operational speeds increase, such a method of monitoring the voltage level of the MDIO becomes less reliable.
It is as indicated in the foregoing, generally the object of the invention to relieve this difficulty primarily in the circumstances of a station management entity and a physical layer device in communication according to the IEEE standard 802.3, clause 22, but in analogous circumstances the invention may have a more general utility.
In essence, the invention relies on the fact that the xe2x80x9cset up and holdxe2x80x9d times for the data are quite narrowly defined relative to the clock and there is accordingly sufficient time, particularly after the last possible clock transition relative to the last address bit before the turnaround time, to drive the management data input/output bus high if necessary. This ensures that the time constant associated with the pull-up resistor is no longer a limitation on the speed of operation and therefore facilitates the detection of the presence of a physical layer device operational at higher data rates than would otherwise be feasible.
The foregoing and other features of the invention will be further explained with reference to a specific example and the accompanying drawings.